We, too, are charter members of the no-hookups-club. I use 4x300Ah 6v batteries. As far as I know, it makes no difference whether you use a 6v battery or a 12v battery so long as you end up with the number of ampere hours you need. People started using 6v golf cart batteries as they were an easy way to find true deep cycle batteries, as opposed to starter batteries.
(For the technically inclined, there are neither 6v nor 12v batteries, only 2v batteries, either three or six to the case.)
So, connecting my 6v batteries in series/parallel, I end up with a 12v, 600Ah battery bank which gives me 300Ah usable.
The type of battery you use is VERY important. For camper use, you want true, deep cycle batteries. That is big, thick plates and lots of acid.
AGM batteries are more expensive but can offer advantages of less maintenance, better vibration resistance, and the option of mounting sideways.
When you start getting over 400Ah, you really need solar or shore power to assure a full charge. Even with the special wiring on my truck, it is hard to drive long enough to fully recharge a big battery bank.
